A Photonic Crystal (PC) is a defined as a porous dielectric composite structure with modulated refractive indices, which control the interaction of light and matter. This interaction is dependent on both the geometry of the crystal as well as the materials parameters. For a Photonic Band Gap (PBG) to exist within a PC, the allowed photonic bands must not overlap for some area, this area is defined as the band gap. A complete 3-D PBG is a gap in the allowed quantum states across all propagation directions and polarization modes. The band gap position and gap width may be easily modified to yield desired photonic properties by varying crystal parameters, such as: structural geometry, crystal lattice dimensions, or contrast of indices of refraction between the composite material and vacuum/air. The ability of a band gap with selected defects to tightly control the propagation of light is attractive for optical waveguides, optical signal processing, telecommunications, superlenses, sensors, and many other applications. The ability to modify the band structure, thus modify the density of states, and thus the thermal emission spectra of a PC material, makes PCs attractive for lighting and ThermoPhotoVoltaic (TPV) applications.
Many 3D PC structures exhibit PBG properties, including: inverse opal, woodpile, diamond, Rod Connected Diamond (RCD), gyroid, and others. PBG devices are typically formed from a template and then inverted one or more times to result in the desired geometry and material composition. Layer by layer manufacturing of 3D PBG structures is expensive and time consuming, especially for structures with fine pitch lattice constants for visible applications. Colloidal self-assembly methods are sensitive to defects, which destroy the band gap.
Holographic lithography offers a method to create an entire 3D template that is many lattice constants thick in a single or multiple volumetric exposures. It is suited to a low number of defects and can produce the lattice constants necessary for a visible band gap. Unfortunately, more complex structures, such as the RCD, are defined by the interference of 7 laser beams, projected into a 3 dimensional space. Thus, the optical setup is very sensitive to phase, polarization, amplitude, and positional variations of the laser sources. Therefore, even thermal drift and vibration can result in the formation of an undesired structure since the projected dimensionality shifts, not just the position. Even simpler structures such as the fcc lattice require more than 3 beams, and are thus phase sensitive. Also, when applied to opaque substrates, all beams must originate from the same side of the substrate. Thus a need has arisen to precisely maintain alignment between multiple lasers in a complex 3D configuration.
Physics requires that all objects emit light, with variations in power level and spectra, depending on the material composition and temperature. Quantum Electro Dynamics requires materials with a shaped availability of quantum photonic states produce a shaped emission spectra, relative to that of a black body.
Holographic lithography promises a low cost, high quality opportunity to template PBGs for lighting applications. However, wire filaments are preferred for lighting applications due to their ease of packaging and compatibility with existing bulb manufacturing infrastructure. Convex surfaces are preferred for TPV applications. Current methods are not suited to non-planar substrates or exposure from both sides of the substrate. Thus a need has arisen to template irregular substrates.
The holographic laser sources are index matched with a prism to the photoresist to limit unwanted distortions from reflected beams. Complex multi-beam prisms mating to non-planar substrates are costly. Thus a need for low cost beam delivery has arisen.
A variety of methods to transform a PC template into a PBG are well known, including electrodeposition, Chemical Vapor Deposition (CVD), and other micromoulding methods.